This invention relates to granular activated carbon manufacture, and more particularly to a new and improved process for making granular activated carbon from brown coal treated with concentrated inorganic acid, without the addition of a carbonaceous binder such as pitch, and to a new and improved granular activated carbon made by such process and having properties which make it suitable for use in water and waste water treatment and in other applications.
2. Glossary Of Terms
In order to facilitate a clear understanding of this invention, various terms of art employed herein are defined as follows:
Abrasion number -- is a measure of the resistance of the activated carbon granules to degrading on being mechanically abraded. It is measured by contacting a sample with steel balls in a pan on a machine and shaking the contents for a given time and determining the resultant particle size distribution and hence the mean particle diameter. The abrasion number is the ratio of the final average (mean) particle diameter to the original average (mean) particle diameter (determined by screen analysis) times 100.
Activated carbon -- is carbon which is "activated" by heating to high temperature preferably with steam or carbon dioxide as the gaseous activating agent in producing an internal porous particle structure.
Activating -- means heating coal at high temperatures on the order of about 600 to about 1000.degree. C. in the presence of a gaseous activating agent as is well known in the art.
Adsorption isotherm -- is a measurement of the adsorptive capacity of an adsorbent (viz. granular activated carbon) as a function of the concentration, or pressure, of the adsorbate (viz. N.sub.2) at a given temperature. It is defined as the constant temperature relationship between the amount adsorbed per unit weight of adsorbent and the equilibrium concentration, or partial pressure.
Apparent density -- is the weight per unit volume of homogeneous granular activated carbon. To assure uniform packing of the granules during measurement, a vibrating trough is used to fill the measuring device.
Ash -- is a principal mineral constituent of coal, carbon and pitch. It is normally defined as a weight percent basis after a given amount of sample is reduced to ash.
Average (mean) particle diameter -- is a weighted average diameter of granular activated carbon sample. A screen analysis is run and the average particle diameter is calculated by multiplying the weight of each fraction by its average diameter, adding the products, and dividing by the total weight of the sample. The average diameter of each fraction is taken as the size midway between the sieve opening through which the fraction has passed and the sieve opening on which the fraction was retained.
Brown coal -- is a low rank lignitic type coal which is ranked below sub-bituminous coal, together with and usually slightly below lignite. Typically, in the moist, as received, or as mined condition, it has a moisture content of more than 40% by weight and generally a moist BTU/lb. of less than 8300, and usually is unconsolidated. See ASTM standard D-388-66, wherein BTU wise, "brown coal", as used herein, would be included in the designation "Lignite A" or "Lignite B", but also see former ASTM standard D-388-58, wherein "brown coal", as used herein, is distinguished from and ranked below "lignite", as used herein, by reason of brown coal's usual unconsolidated characteristic.
Charring -- means heating coal at low temperatures on the order of about 175.degree. C. to about 275.degree. C. in the presence of oxygen.
Coking value -- is usually expressed as percent residual carbon obtained when a dry sample of coal, tar or pitch is vaporized or pyrolized for a specific time at a specific temperature that limits the available oxygen supply (ASTM Method D-2416). The coking value, expressed as percent residual carbon, indicates the coke forming properties of the material.
Devolatilizing -- means heating coal at imtermediate temperatures on the order of about 400.degree. C. to about 600.degree. C. in an oxygen-free atmosphere.
Granular activated carbon -- is "activated carbon" which has a particle size, i.e., "mesh", which is not less than about 40 and preferably not less than about 60.
Iodine number -- is the milligrams of iodine adsorbed by 1 gram of granular activated carbon at an equilibrium filtrate concentration of 0.02 N iodine. It is measured by contacting a single sample of carbon with an iodine solution and extrapolating to 0.02 N by an assumed isotherm slope. This number can be correlated with the ability of granular activated carbon to adsorb low molecular weight substances.
Lignite -- is a low rank lignitic type coal which is ranked below sub-bituminous coal, together with and usually slightly above brown coal. Typically, in the moist, as received, or as mined condition, it has a moisture content of more than 20% by weight and generally a moist BTU/lb. of less than 8300, and usually is consolidated. See ASTM standard D-388-66, wherein BTU wise "lignite", as used herein, would be included in the designation "Lignite A" or "Lignite B", but also see former ASTM standard D-388-58, wherein "lignite", as used herein, is distinguished from and ranked above "brown coal", as used herein, by reason of lignite's usual consolidated characteristics.
Mesh -- (or mesh size) is the particle size of granules as determined by the U.S. Sieve Series or the Tyler Series. Usually, this term refers to the sizes of the two screens, in either of the above series, between which the bulk of a sample falls. For example, "8/30 mesh" (or "8 by 30 mesh" or "8 .times. 30 mesh") means that 90% by weight of the sample will pass through a No. 8 screen but will be retained on a No. 30 screen. Alternatively, this term refers to a maximum particle size, such as in defining the fineness of powder material. For example, "65% by weight -325 mesh powder" means that 65% by weight of a given sample passes through a No. 325 mesh screen.
Molasses number -- is calculated from the ratio of the optical densities of the filtrate of a molasses solution treated with a standard activated carbon and the activated carbon in question.
Pitch -- is a black or dark viscous substance obtained as a residue in the distillation or organic materials and especially tars.
Powder -- means a particle size, i.e., "mesh", which is smaller than about 40 and preferably smaller than about 60. The larger the mesh number, the smaller the size.
Surface area -- is the amount of surface area per unit weight of granular activated carbon; it is determined from the nitrogen adsorption isotherm by the Brunauer, Emmett and Teller (BET) method, and it is expressed in m.sup.2 /gram.
3. Prior Art
Granular activated carbon is particularly useful in water and waste water treatment not only because it is highly effective in purifying the intake, as well as the effluent from municipal and industrial systems, but also because it can be regenerated for repeated use. However, in order to accomplish these objectives it must possess certain properties, namely, a minimum surface area of about 900 m.sup.2 /gram for adequate adsorption capacity, a minimum iodine number of about 900 for adequate adsorption of low molecular weight substances, a minimum Molasses number of about 200 for adequate decolorizing, a maximum ash content (by weight) of not more than about 12%, and preferably not more than about 8%, for purity, a minimum abrasion number of about 70 and preferably not less than about 80, for adequate hardness in maintaining granular integrity in use and in regeneration, and a minimum apparent density of not less than about 0.46 gram/cc, preferably about 0.48 gram/cc, for obtaining the dense, closely packed beds and columns needed in waste water treatment.
These properties can be obtained by making granular activated carbon from bituminous coal and from sub-bituminous coal, such as disclosed in both of my aforesaid copending applications Ser. Nos. 884,084 and 818,699, and my aforesaid patent, as well as from brown coal, such as disclosed in my copending Serial No. 856,881, by dilute inorganic acid treatment of brown coal, which is considerably cheaper, as the starting material, than bituminous coal, and usually even cheaper than sub-bituminous coal. However, until the present invention, it is not known that anyone else has accomplished this by concentrated inorganic acid treatment of brown coal, without the addition of a carbonaceous binder.
In my aforesaid copending Serial No. 856,881, it was found that this objective could be accomplished by dilute inorganic acid treatment of brown coal, as noted above. However, this involved treating the coal granules with a dilute inorganic acid, washing off the acid and at least partially drying the granules. In one case, he granules were dried partially to a moisture content of about 10 to about 25% by weight without the addition of a carbonaceous binder such as pitch. In the other case, the granules were dried thoroughly and thereafter mixed with about 5 to about 15% by weight of such binder. In the present invention, the process is simplified by treating the coal granules with a small amount of concentrated inorganic acid, without the addition of a carbonaceous binder, while at the same time controlling the moisture (including acid) content of the powder formed from the treated granules, in order to permit proper compacting and further processing to produce hard granular activated carbon, wherein the moisture (including acid) content of the powder is used as a temporary binder, all as described in detail below.
Moreover, when so using bituminous coal, it has been found necessary not only to mix in pitch but also to char the granulated mixture prior to the devolatizing and activating steps. Otherwise, because of the high coking tendency of the preferred bituminous coals, the granules fuse together during devolatization and are thereby rendered unsuitable both for proper activation and for obtaining the aforesaid desired properties. Likewise, in working with various bituminous coals, it has been found that this charring step is necessary, whether or not the granules have been treated with either a concentrated inorganic acid or a dilute aqueous solution of inorganic acid prior to charring. While it has been found that such dilute inorganic acid treatment does have a substantial beneficial effect on at least the aforesaid properties of granular activated carbon produced from low rank bituminous coal, as disclosed in my aforesaid copending Ser. No. 818,699, and that such dilute and concentrated inorganic acid treatments do have a substantial beneficial effect on both the overall yield and properties of granular activated carbon produced from sub-bituminous coal, neither treatment has little, if any, beneficial effect upon either the overall yield of the resulting granular activated carbon or the aforesaid properties desired, when working with a highly coking, preferred bituminous coal.
Furthermore, it has been found herein that granular activated carbon of the aforementioned properties can not be produced from brown coal when such coal is not subjected to such concentrated acid treatment or charring, despite the fact that such coal usually is not coking at all. Although it has been found herein that brown coal can be charred without such concentrated acid treatment, to produce granular activated carbon, the yield is very low and the properties, at best, are below the minimum acceptable for granular activated carbon suitable for use in water and waste water treatment and in other applications. As a matter of fact, it has been found herein that the charring step, originally thought necessary for so processing brown coal, can be eliminated, and that if appropriate concentrated inorganic acid treatment is employed, this results in significant increases not only in yield, but also in the desired properties, without the need for a carbonaceous binder.
Moreover, and surprisingly enough, it has been found that such concentrated acid treatment does not produce an acceptable granular activated carbon from lignite, at least the particular variety treated.
There have been some attempts in the past, as set forth in U.S. Pat. No. 2,049,931, to produce activated carbon from brown coal or lignite by the addition thereto of a large amount of concentrated inorganic acid, but following the teachings of this patent does not allow one to make a hard granular activated carbon, rather only powdered activated carbon. A more recent attempt to make a commercially successful activated carbon from low grade coal, such as bituminous, sub-bituminous and lignitic coals by dilute acid treatment is disclosed in U.S. Pat. No. 4,014,817. However, the purpose of this treatment is to increase the sorbant properties of the product by the removal of magnesium and calcium salts, and the patented process still requires heat oxidation or charring of the coal following acid treatment. This heat oxidation adversely affects yield, and tests have shown that the granular product of the patent, while dense and sorptive, does not possess the hardness required for maintaining granular integrity in use and in regeneration. Thus, the patented product is not suitable for use in water and waste water treatment.
Other more recent attempts to produce activated carbon from Australian brown coal are indicated in U.S. Pat. Nos. 3,998,604 and 4,039,473.
In the former, the process actually disclosed merely provides an acid environment which consists in the separation of sand and clays present in the coal for demineralization purposes (i.e., ash removal) only. Although this patent briefly mentions further treatment for preparing activated carbon, it does not specify either how this is to be done or the quality and type of product ultimately to be produced.
In the latter, the process disclosed is directed to the production of activated carbon by first converting the coal into a potassium coal, carbonizing the same and then acid leaching to remove potassium, followed by activation. While the activated granular product is said to have high surface area, the hardness, density and integrity thereof are not even mentioned.
Hence, neither of these two patents suggests an operational process for successfully producing a hard and dense granular activated carbon suitable for use in water and waste water treatment.